Additional args to be passed to `httr::GET()`

Hi Hadley,

this is my first pull request ever - so please don't bite ;-)

## Reason for modification
I needed to pass along altered user agent information for the underlying `GET` request in `html()`

## Actual modification
In order to avoid this

```
require("httr")
content(GET(<url>,  user_agent("Mozilla/5.0")))
```

I modified `html()`: it now accepts `...` to pass additional arguments along to `httr::GET()` for this to work

```
html(<url>, encoding = NULL, user_agent("Mozilla/5.0")
```

As making use of `...` involves the need to explicitly specify `encoding = NULL` in calls to `html()` (unless making `...` the second argument in `html()`), you might want to go with the second (commented-out) implementation of `html()` featuring an actual formal argument `httr_config` for the things to pass to `httr::GET()`.

## Unit test
I also added a short unit test for a German website that throws a `403` error for the default user agent.

Best regards and thanks for the cool package!
Can you please test against http://httpbin.org?